1. Field of the Invention
The present invention relates to an optical-receiver system and, in particular, to a temperature-compensating device for compensating a performance loss associated with a temperature in an optical receiving device, which employs an avalanche photo diode (APD).
2. Description of the Related Art
When light is incident into an optical diode and a reverse-bias voltage is applied gradually thereto in an increasing fashion, electrons are produced and accelerated in a high electric field, causing atoms to collide with each other. As a result, an avalanche phenomenon occurs in which new electrons and holes are produced. An avalanche photo diode (APD) is used for converting an optical signal into an electrical signal and as a light-receiving element in an optical communication system. Further, the APD has a characteristic suitable for use in a high-speed digital line.
Moreover, the APD has its own gain control and is capable of detecting a very low level of light. Therefore, the APD is preferred by developers who seek optimum sensitivity in a fiber-optic receiver.
However, in order to obtain a sufficient level of such an internal gain, it is necessary for the APD to be supplied with a high bias (30 to 40V). Yet, the APD has temperature-sensitive gain characteristics. Therefore, in an optical-receiving system that employs an APD not only for obtaining a constant gain of the APD but also for supplying a high voltage, a compensation circuit is necessary for compensating the change in temperature. In addition, the APD has some drawbacks in that due to an electric-current amplification effect caused by the avalanche phenomenon, its signal-to-noise (S/N) ratio and bias voltage tend to be high and highly temperature dependent.
Currently, a control circuit that incorporates an APD uses a power switching technique known as PWM (Pulse Width Modulation), which serves to amplify the voltage at a high level with a minimum current from a power generator.
For example, “MAXIM1771” chips manufactured by Maxim Integrated Products Inc., of 120 San Gabriel Dr., Sunnyvale, Calif., USA, are commonly used as a core chip for amplifying the voltage. The voltage level of this chip is changed in accordance with a change in the temperature. The change of the voltage level in accordance with the change in the temperature is controlled by the voltage level applied to the feedback terminals of the chip. As such, the range of voltage change can be controlled and determined by the feedback terminals. Here, a temperature-compensating circuit is formed by combining a resistance used for controlling the slope indicative of the voltage change and a resistance connected to a thermistor incorporated in the APD used to provide a linear behavior of resistance in accordance with the temperature.
FIG. 1 shows a configuration of a conventional APD optical-receiver system. As shown, the conventional APD optical-receiver system comprises: a voltage-generator section 11 for generating and outputting a high reverse bias using a low-voltage input; an APD-receiving section 13 provided with an APD 131 and a thermistor 132 for sensing the temperature change in the APD; and, a temperature-compensating section 12 for compensating a decrease in gain caused by the temperature change in the APD 131, so that the voltage generator 11 can generate a linearized output voltage.
FIG. 2 shows the configuration of the conventional APD optical-receiver system shown in FIG. 1 in more detail.
Referring to FIG. 2, the voltage-generator section 11 comprises a “MAXIM 1771” chip mentioned earlier and uses a PWM power switching technique; a voltage-generator section 11 generates a reverse bias in the range of 30V to 40V responsive to the input voltage of 5V to 8V. In operation, the APD 131 connected to the output Vo of the voltage-generator section 11 suffers from a decrease of 0.2%/° C. in the gain factor as the temperature increases. Therefore, in order to maintain the receiving sensitivity at a constant level, it is necessary to increase the reverse bias at the same rate as the rate of temperature increase.
In order to increase a reverse bias in accordance with the temperature, the resistances of the temperature-compensating section 12 are connected to the thermistor 132 of the APD in parallel, so that it is possible to tune the change rate of the resistance in accordance with the temperature change to generate an output voltage Vo at a constant level. This can be achieved by combining the selected resistances R2, R3, and R4 of the temperature—compensating section 12 and the resistance RT of the thermistor 132 of the circuit, as a result of which it is possible to control the range of the output voltage Vo of the voltage-generator section 11 in the circuit according to following Equation 1, which defines the relationship between the reference voltage Vref and the output voltage Vo.
                                          V            o                    =                                    V              ref                        ⁢            s            ⁢                                        J                        ⁢                          R1                              R2                +                R3                +                                  (                                                            R4s                      ⁢                                                                                          ⁢                      RT                                                              R4                      +                      RT                                                        )                                                      ⁢                                                          ;                            K                                      ⁢                                                      Equation        ⁢                                  ⁢        1            
That is, in the above example, a voltage generation is provided in accordance with power switching by the voltage generator 11, which incorporates the “Maxim1771” chip, and because the voltage generation consists of a voltage amplification in accordance with the ratio defined by Equation 1, the related resistances R2 and R3 control the slope gain of the voltage amplification and the temperature linearization of the APD in accordance with the change of temperature, respectively. Here, the change and the amplification of the voltage determined by R1 and R2 are shown in FIGS. 3a and 3b. 
FIG. 3a shows the relationship between the parallel resistance value RT and the temperature. In particular, the X-axis indicates the temperature of the APD 131 and the Y-axis indicates the resistance value of the thermistor 132. FIG. 3b shows the relationship between the output voltage Vo and the temperature. In particular, the X-axis indicates the temperature of the APD 131 and the Y-axis indicates the output voltage Vo of the entire circuit.
Referring to the operation of the conventional APD optical-receiver system in more detail, it is necessary to consider the entire slope of the voltage amplification and the voltage magnitude of the resistance R2 in the temperature compensator 12. It is also necessary to tune the linearity of the resistance change in accordance with the temperature change by changing the resistance R3 of the temperature compensator 12, which is connected to the thermistor 132 in parallel.
However, each APD 131 has a different reverse-bias value needed for obtaining an inherent gain factor, and the change of voltage is sensitive to the change of resistance values of R2 and R3 of the temperature compensator 12. Therefore, it is necessary to establish the resistances R2 and R3 of the temperature compensator 12 whenever the APD 121 is replaced in a circuit board and whenever the ambient temperature is changed, in order to maintain a constant receiving sensitivity, i.e., in order to allow the output voltage to be linearized. If the resistance R2 is made as a variable one in order to maintain the receiving sensitivity constant as shown in FIG. 2, it may be possible to meet under the macroscopic condition of 0.2%/° C. at the initial and the final ends of the entire temperature-changing region. However, it is impossible to obtain a voltage-change rate of 0.2%/° C. for the entire temperature spans. Also, it is necessary to debug each resistance value of the R2 and R3 even if such a voltage-change rate can be obtained. Furthermore, it is difficult to linearize the resistance value as Equation 1, which is needed for making the resistance value by an analogue method to be expressed as a fractional equation.